In volume rendering for displaying technical or medical images 3-dimensionally on a display screen, highlighting by specular reflection on the surface of a semi-transparent object constitutes an important element for the operator's visual perception of the depth and surface features of a semi-transparent object.
In volume rendering, a stereoscopic image is usually generated by shading volumes by using a suitable shading model, weighing the volumes according to opacity, and projecting the volumes thus shaded according to the weights onto a screen.
By the conventional ray tracing method for volume rendering, a stereoscopic image is formed by projecting data about color- and opacity-assigned volumes obtained at a sampling point onto pixels on a screen.